Modelagem e validação Monte Carlo do acelerador Halcyon™ para caracterização do Beam Stopper

Abstract

The Varian Halcyon™ linear accelerator, featuring an O-Ring architecture and fully jawless collimation by a dual-layer multileaf collimator, has a high-atomic-number beam stopper positioned opposite the isocenter, whose radiation-protection function still requires quantitative characterization. This work aimed to develop, validate, and apply a geometric model of the Halcyon™ treatment head in the MCNP6.2 radiation transport code, based on the stochastic Monte Carlo method, in order to characterize the performance of the beam stopper in structural shielding and in patient dose. The model, comprising the target, the primary collimator, the dual-layer multileaf collimator, and the lead–antimony alloy beam stopper (11.34 g/cm³), was validated against the clinical commissioning data of the Eclipse treatment planning system. The gamma index was used, with the 3%/3 mm and 2%/2 mm criteria, on percentage depth-dose curves and lateral profiles for seven field sizes, from 2 × 2 to 28 × 28 cm². The validation showed satisfactory agreement between the model and the clinical data, with passing rates above 95% under the 3%/3 mm criterion in most configurations and deviations expected only for the smallest field sizes. Once the model was validated, the attenuation of the primary beam by the beam stopper and its contribution to the primary barrier of the bunker were estimated. A real clinical prostate volumetric modulated arc therapy (VMAT) plan was then reconstructed from the accelerator's trajectory log and the ICRP 145 AM anthropomorphic phantom, in order to quantify the peripheral and surface dose associated with backscatter from the component. The beam stopper proved to be an effective attenuator of the primary beam transmitted along the central axis, equivalent to 2.58–2.98 tenth-value layers (TVL). Its effect on patient dose was found to be real but clinically negligible: below 1.4%, restricted to distal cortical bone structures outside the treatment field, corresponding to a small increase in dose due to backscatter and with no statistical significance in the irradiated regions. It is concluded that the beam stopper acts essentially as a shielding element, with trivial influence on the therapeutic dose, and that the developed model constitutes a validated tool for dosimetric and radiation-protection studies in accelerators with an architecture similar to that of the Halcyon™.